1. Field of the Invention
The invention is directed to roof construction and suspended ceiling system and, more particularly, to a fire rated suspended ceiling system.
2. Description of the Prior Art
This invention concerns a roof structure and suspended ceiling system consisting of a roof with a watertight outer layer and a layer of thermal insulation beneath it, resting on a metal roof-supporting surface that has a vapor barrier, plus a suspended ceiling that is suspended from the roof-supporting surface with the ceiling tile supported by suspended rails.
There is already a known roof structure and suspended ceiling system in which the rails that support the ceiling tiles of the suspended ceiling are suspended from a metal corrugated roof-supporting surface. The roof-supporting surface has a layer of sheet gypsum on its upper side and a layer of thermal insulation such as mineral wool above it, and this is sealed by a watertight outer layer. The roof-supporting surface forms a vapor barrier (German Patent Application No. 2,705,032). It is also known that a layer of asphalt can be applied to the roof-supporting surface to form the vapor barrier.
It has been found that in the event of fire, the fire resistance of this system does not meet the 90-minute requirement, despite the layer of sheet gypsum on the roof-supporting surface that forms a heat sink, because the metal roof-supporting surface reaches excessively high temperatures too rapidly.
To prevent this rapid heating of the metal roof-supporting surface in the space beneath the suspended ceiling in the event of fire, a layer of mineral wool could be appplied as thermal insulation to the ceiling tiles of the suspended ceiling, but the disadvantage of this arrangement is that in unfavorable weather conditions, the dew point in the space between the suspended ceiling and the roof-supporting surface could shift, so the suspended ceiling would be exposed to moisture, and this must be avoided at all costs.
To keep the dew point outside the space, even in very cold weather, the layer of thermal insulation on the roof-supporting surface would have to be increased considerably, so that increased cost due to this method would result in a very expensive roof structure and suspended ceiling system.